1. Field of the Invention
The present invention concerns a code division multiple access (CDMA) communication system, and more particularly a hard handoff system for handing off the service of a mobile phone from one base station to another base station and method therefor.
2. Description of the Related Art
Generally, in a CDMA communication system, when a mobile phone is moving from the service area served by one base station to the adjacent service area served by another base station, there is required a hard or soft handoff operation to continuously serve the mobile phone without interruption of the communication. The soft handoff is achieved without changing frequency channel, frame offset, or pseudo noise (PN) offset, etc. while the hard handoff requires changing of at least one of them.
Typically, when a mobile phone is moving from a first cellular service area served by corresponding base station to the second cellular service area served by a different base station, the handoff operation is started by a control station in the overlapped zone common to the first and second cellular service areas. The control station carries out a soft handoff or a hard handoff according to predetermined situations. Namely, the control station carries out the soft handoff if the traffic channel employed by one base station is available for another base station, or else the hard handoff is carried out. A soft handoff technique for use in a CDMA cellular telephone system is described in U.S. Pat. No. 5,101,501 to Klein S. Gilhousen, et al., entitled Method And System For Providing A Soft Handoff In Communications In A CDMA Cellular Telephone System. U.S Pat. No. 5,594,718 to Lindsay A. Weaver, Jr., et al. entitled Method And Apparatus For Providing Mobile unit Assisted Hard Handoff From A CDMA Communication System describes both a soft handoff technique and a hard handoff technique for use where neighboring cell sites may utilize either a CDMA system or an FM system. U.S. Pat. No. 5,649,000 to Dong-Wook Lee, et al., entitled Method And System For Providing A different Frequency Handoff In A CDMA Cellular Telephone System proposes a system for use in an area where a soft handoff is not possible, e.g., a CDMA system which includes a FDMA (frequency division multiple access) scheme, and thus utilizes a hard handoff technique.
The soft handoff uses a diversity combining technique employing a rake receiver, and the adjacent base station is added to the presently serving base station whose transmission power strength is large enough to be effective, so that there occurs hardly a problem in the communication between the mobile phone and the control station during handoff. Further, the soft handoff is carried out between two channels of the same frequency, thus securing reliability of the handoff. An example of a diversity receiver is provided in U.S. Pat. No. 5,109,390 to Klein S. Gilhousen, et al., entitled Diversity Receiver In A CDMA Cellular Telephone System.
Compared to the soft handoff, the hard handoff suffers from an inherent drawback because the handoff is over a different frequency on an adjacent traffic channel. In this case, the diversity is removed as soon as the service channel of the mobile phone is changed to the different traffic channel having the different frequency, and therefore the receiving quality is degraded compared to the previous communication environment. In addition, the power strength received by the mobile phone differs before and after changing the frequency channel even at the same position. Therefore, the power strength of the presently serving base station becomes weak making it undesirable for use in the communication between the mobile phone and the control station.